High Speed Downlink Packet (HSDPA) is a further development of WCDMA, enabling considerably higher bit rates in the downlink. In order for the network to achieve these improvements, some indication of the present channel quality has to be provided to the network. On the basis of this information, appropriate preparations can be performed at the network in order to provide for the required data transfer in the downlink. For this purpose Channel Quality Indicators (CQIs) are used. The aim of forwarding a CQI from a User Equipment (UE) to the network is to use the information obtained from the CQI, and to allow the network to perform various tasks, such as e.g. channel dependent scheduling, link adaptation and downlink power allocation, on the basis of the retrieved information.
In WCDMA the RRC states IDLE, CELL_PCH, URA_PCH, CELL_FACH and CELL_DCH are used. FIG. 1 is a schematic illustration of a typical scenario for CQI forwarding involving a UE 100 and a base station 101 which are involved in an information exchange.
In a first step 1:1 base station 101 transmits reference signals to UE 100. The reference signals are used by UE 100 to determine the present downlink channel quality. After having determined the downlink channel quality on the basis of the received reference signals, UE 100 sends one or more CQIs to the base station 101, as indicated with a second step 1:2. The base station 101 uses the content of the CQI reports for performing tasks, such as e.g. link adaptation, resource allocation, power control, and scheduling. After an adequate processing of the obtained information at the base station 101, UE 100 is informed of the result of the performed task, e.g. the resulting link adaptation, in a next step 1:3, and subsequent to the respective preparations, downlink transmission is executed via the allocated resources, as indicated with a final step 1:4.
In CELL_DCH state downlink data transmissions and resource allocation are performed via HS-PDSCH and HS-SCCH channels. In HSDPA, which is a release 5 feature, the uplink in CELL_DCH state HS-DPCCH is used for carrying CQI and ACK/NACK to the network.
A reported CQI normally depicts downlink channel quality corresponding to a certain transport block size when using a certain modulation and coding that could be received by a UE with 10% block error rate. This type of mapping between CQI and transport block sizes for WCDMA is defined in 3GPP technical specification 25.214. Thus, a CQI reported from a UE to the network enables the network to select an appropriate Transport Block (TB) size for upcoming transmissions. Thanks to the CQIs, the downlink transmission rate therefore can be optimized and enhanced.
The TBs may range between [1,31], with a resolution of 1 dB, where a TB size that equals 1 is the smallest size and a TB size that equals 31 is the largest size that can be used for downlink transmissions. However at present the largest TB size used is 30, while a TB of size 31 may be used in the future. Mapping of a reported CQI to TB size for different modulation and coding scheme is specified in further detail in the 3GPP standard TS 25.214.
CQI reporting is typically executed in a periodic manner, which is also referred to as the CQI feedback cycle, where the reporting can be adjusted by the network through higher layer signaling. The CQI feedback cycle is expressed in Transmission Time Intervals (TTIs). At present the possible CQI feedback cycle values are 0, 2, 4, 8, 10, 20, 40, 80 and 160 TTIs. When uplink discontinuous transmission is used it is possible to temporarily switch off CQI reporting in order to reduce the uplink interference.
In release 5 the UE receiver performance requirements are solely based on the baseline classical rake receiver of the UE. The corresponding performance requirements are commonly termed and specified as minimum performance requirements in the 3GPP technical specification TS 25.101.
Also in release 6 and beyond the enhanced UE receiver performance requirements have been specified. In order to fulfill these requirements and to pass the corresponding conformance tests, the UE will have to implement advanced receiver features, such as e.g. receiver diversity, a chip level equalizer and/or a generalized rake (G-rake) receiver. Evidently the goal of the specification of these enhanced requirements is to significantly boost the downlink bit rate.
In WCDMA terminology UE receiver performance requirements for various advanced receivers have, until now, been specified as enhanced receiver type 1, type 2, type 3 and type 3i. However, the enhancement receiver performance specification does not preclude the UE vendors to implement advance receivers beyond the specified enhanced requirements.
As of today a HSDPA capable UE normally reports its category in terms of e.g. a maximum number of codes, or bits, in one TTI when this information is forwarded to the network. However, the UE does not report any of its enhanced receiver capabilities, such as e.g. the enhanced receiver type. As a consequence, the network is completely oblivious of the type of enhanced receiver that is implemented at the UE. In order to obtain optimum performance it is, however, of paramount importance that different functions, such as e.g. the scheduler, at the base station are able to fully make use of the enhanced UE receiver capabilities. A UE having an advanced receiver will obtain a better estimation of the downlink Signal-to-Interference and Noise Ratio (SINR) estimation, compared to if a baseline rake receiver is used.
The CQI is fundamentally derived from the SINR, which in turn is estimated on the basis of the Common Pilot Channel (CPICH). Hence the reported CQI implicitly depicts the actual receiver performance. This means that a UE equipped with a more advanced receiver will be able to report relatively high CQIs, which in turn will allow the network to schedule a higher data rate to the UE, and, thus, a higher performance will be obtained from the UE.
In release 7 a new feature, often referred to as enhanced CELL_FACH state, have been introduced. This feature allows mapping of generally low bit rate data, such as e.g. paging or small packets, in low RRC activity states, such as e.g. the IDLE mode, CELL_PCH, URA_PCH or CELL_FACH state, on to a HS-DSCH transport channel. The main advantage with such a feature is that in any of these low activity states the paging or data can be swiftly transmitted to the UE. This is because the scheduling on HS-DSCH is done at the base station and the HS-DSCH is shared between multiple UEs on a TTI basis, which typically has a periodicity of 2 ms.
In addition, according to the state of the art technology, transmission between a UE and the network in enhanced CELL_FACH state is characterized by the fact that a UE receives the scheduling information via HS-SCCH, i.e. in a same manner as in the CELL_DCH state. However, in enhanced CELL_FACH state there is no HS-DPCCH channel available for a UE to report a CQI or ACK/NACK, and, thus, no link adaptation or channel dependent scheduling will be possible in this state. Furthermore, the network blindly transmits a fixed number of HARQ transmissions, i.e. a first transmission and, whenever necessary, up to a specified number of retransmissions. HARQ combining is, however, possible at the UE.
In release 7 and beyond, however, use of the High Speed Downlink Shared Channel (HS-DSCH) is possible also in any of the low activity RRC states, i.e. in idle mode, URA_PCH, CELL_PCH and CELL_FACH states. This option allows a mapping of PCH and FACH transport channels to the shared channel, i.e. the HS-DSCH. However, in order to minimize the uplink load, a UE that is in any of these states is not allowed to report any CQIs to the network.
Further improvements have been specified in release 8 in which CELL_FACH in the uplink has been improved by enabling E-DCH activation where, for example, the base station controls resources for common E-DCH and the required downlink control channels, i.e. Fractional Dedicated Channel (F-DPCH), E-DCH HARQ Acknowledgement Indicator Channel (E-HICH) and E-DCH Absolute Grant Channel (E-AGCH). Common E-DCH configurations are broadcasted on BCCH, which can transmit short packets using E-DCH. However, unlike in CELL_DCH, the E-DCH establishment phase in enhanced CELL_FACH is much shorter, which results in a faster call setup, a faster packet transmission and in a reduced overall latency.
As already noted above, CQI reporting is currently not done in enhanced CELL_FACH state. This means that the network can neither perform channel dependent scheduling, nor link adaptation, such as using an adaptive modulation or coding scheme when it is in this state. Furthermore, downlink power control on HS-PDSCH, or on any other downlink physical channel, cannot be accurately executed in this state due to the lack of CQI reporting.
Another major repercussion from the lack of CQI reporting in the enhanced CELL_FACH state is that there is no motivation for network operators to specify enhanced receiver requirements. As a matter of fact, up to date only minimum performance requirements for enhanced CELL_FACH scenario have been specified.
It is important to note that advanced receiver performance requirements are specified for each physical channel separately. On the one hand, the implementation of an advanced receiver boosts user throughput, but on the other hand it also increases cost and UE battery power consumption. Therefore, advanced receivers will work strictly on those channels for which performance requirements exist. This means that the enhanced receiver requirements for HSDPA in the CELL_DCH state fail to imply that enhanced receivers are also implemented for HSDPA reception in the enhanced CELL_FACH state.
As indicated above, the main problem with the current enhanced CELL FACH state feature is that downlink scheduling is done without any knowledge of the downlink radio conditions, due to the lack of CQI reports. This means that channel dependent scheduling and link adaptation is not possible, and thus as a result from this deficiency the throughput performance will be significantly poor. Using a fixed number of HARQ re-transmissions, e.g. 3 or 4, without taking any regard to the present radio conditions lead to wastage of radio resources. In fact the main bottleneck with this approach is that presently there is no reason or motivation for any UE vendor to implement an advanced receiver for data reception in the enhanced CELL_FACH state. Any sort of CQI reporting would, however, motivate the implementation of advanced receivers at the UE.
One obvious and straight forward solution to the deficiencies mentioned above is to follow the conventional path by defining a normal CQI reporting scheme as is presently done in the CELL_DCH state also for the enhanced CELL_FACH state. A primary concern with this approach is, however, that in enhanced CELL_FACH state, where no UE specific channel is in operation, the conventional CQI reporting scheme may lead to unsustainable load on the RACH channel, which is presently the only uplink transmission mode in enhanced CELL_FACH state, as specified in release 7. In addition, the CQI reports are not really needed as frequently in the enhanced CELL_FACH state as in the CELL_DCH reception scenario.
In the Swedish patent application 0602299-0 it is proposed that a CQI threshold, that can be either cell- or UE specific, is transmitted from the network to a number of UEs, and that each UE whose CQI is below the CQI threshold needs to report a new CQI to the network. One deficiency with this solution is that even for a dynamic forwarding CQI mechanism, the probing and decision making of a forwarding of a new CQI by the network to a UE will result in a delay. Secondly if radio conditions change quickly, several UE's may rapidly move between good and bad conditions, with the severe risk that many users will start to report the CQI more frequently, thereby increasing the uplink load.
It is therefore a desire to enable a regulated CQI reporting that does not have a negative effect on the uplink load.